I can't seem to get off this Nancy Drew topic. Talking to my aunt last week made me wonder what mint-condition vintage Nancy Drew books might be worth. A visit to abebooks.com yielded some surprises. Currently for sale: a first edition of The Password to Larkspur Lane (1933), in dust jacket, for sale for $6500. Now, there's no guarantee it will sell at that price. But that's what the dealer is asking. The copy is described as in "very good" condition, which any collector will tell you is quite different from "mint" or "as new" or "fine."
The next most expensive Nancy Drew for sale right now is a first edition of The Clue in the Diary (1932), also in "very good" condition, for $2500.
I wondered if I would find any Nancy Drews claiming to be signed by the author, since Carolyn Keene didn't actually exist. I didn't find any, but I did find a book signed by Mildred Wirt, one of several authors who used the Keene pseudonym. She published, under her own name, Ruth Darrow in the Fire Patrol (1930), and there's a signed copy available for $650 right now.